1. Field of the Invention
The present invention relates to a picture element density converting apparatus, and more particularly, to a picture element density converting apparatus for binary images in which a pseudo half-tone processed image and characters or line drawings are present in a mixed state.
2. Description of the Prior Arts
In a case where the resolution of an image reading device or an image printing image differs from that employed on communications in a facsimile system or the like, or in a case where enlargement or reduction of image data is performed by means of an image editing device or the like, the density of the picture elements of an image must be converted.
Conventionally, various methods, such as the SPC method, the logical OR method, the 9 division method, the projection method, the linear interpolation method and the distance inverse proportion method, have been proposed as pixel density conversion methods for such binary images. These methods are mainly employed for characters or line drawings.
In the case of reduction in the density of picture elements of information represented by a binary signal, the thinning out method is used. In this method, the original pixels are thinned out at a fixed ratio so as to reduce the number of original pixels.
In the case of reduction in the pixel density, the logical OR method is also employed. In this method, for example, a total of 4 pixels, 2 pixels in the vertical direction and 2 pixels in the horizontal direction, are referred to, and, if the number of black pixels is 1 or above, these 4 pixels are replaced by 1 black pixel while, if all 4 pixels are white pixels, they are replaced by 1 white pixel.
However, the thinning out method has a disadvantage in that thin lines drawn on an original at a width corresponding to one pixel may disappear or be broken due to loss of the pictorial information held by the discarded pixels. The logical OR method has a drawback that it is not suited to pseudo half-tone processed images which have been processed by the dither method, the error diffusion method or the like, because the half-tone of these images is expressed by a ratio of the area of the black area to that of the white area, but this logical OR method increases the ratio of the area of the black area in a converted image.
Japanese Patent Laid-Open No. 281673/1987 discloses the conversion method suitable for binary images obtained by the dither method. In this method, multi-level data which is to be subjected to the dither process is estimated using an average value filter or the like. Pixel density conversion is conducted on the obtained multi-level data, and the converted data is binarized again. However, it is difficult to estimate multi-level data which is to be subjected to the quantization process with a high degree of accuracy. Filtering may reduce the resolution or change a tone. Furthermore, this process cannot be applied to an image which contains characters or figures.
In a facsimile system or the like, since disappearance of thin lines is a more serious problem than blurring thereof, the logical OR method is mainly employed. However, in this method, blurring occurs and lines are thereby thickened when the conversion factor is in the vicinity of 1 or in the case of enlargement.
Among the above-described conversion methods, the interpolation method, such as the linear interpolation method, and the projection method are known as the conversion method in which blurring or disappearance of the thin lines is little.
However, in both the interpolation method and the projection method, disappearance or blurring of the thin lines occurs in the case where the conversion factor is low.
More specifically, when the conventional conversion methods mainly used for characters or line drawings are applied to images which have been subjected to the psuedo half-tone process, such as the dither method, moire or cyclic lines are generated and the quality of images therefore deteriorates in either of these methods and particularly in the case of conversion by a factor other than an integer. When the conventional conversion methods mainly used for images which have been subjected to the pseudo half-tone process are adopted to characters or line drawings, degradation of the image quality, including reduction in the resolution and disappearance of thin lines, is great.
In the case of the image processed by the organized dither technique method and in a case where the dither matrix is known, it has been proposed to process an image in which pseudo half-tone processed images and characters or figures are present in a mixed state by excellently estimating an original multi-level image (Japanese Patent Laid-Open No. 157468/1987). However, this method cannot be applied to images which have been subjected to the pseudo half-tone process by an unspecified method and whose dither matrix is unknown or to those which have been binarized by the conditional determination dither method.
Thus, the conventionally proposed conversion methods do not assure excellent conversion of the pixel density of an image in which images which have been subjected to the pseudo half-tone process by an unspecified method and characters or figures are present in a mixed state.